The present invention relates to an A/D converter using a charge transfer device (CTD), especially a charge-coupled device (CCD).
A traveling-wave converter device is known as a high-speed A/D converter. A typical example of the basic circuit block of the traveling-wave converter device is shown in FIG. 1.
Referring to FIG. 1, an input analog signal VIN is supplied to one input terminal of a comparator 3, and a voltage VR half of the full-scale input analog signal VIN is applied to the other input terminal of the comparator 3. An output from the comparator 3 serves as an MSB (Most Significant Bit) which is then stored in a memory 6. At the same time, the MSB is supplied to a D/A converter 8 in response to a timing signal .phi.. The D/A converter 8 generates a 3/4 or 1/4 voltage of the full-scale input analog signal VIN in accordance with data of logic "1" or "0" from the memory 6. An output from the converter 8 is supplied to one input terminal of a comparator 4. A delayed input analog signal VIN is supplied to the other input terminal of the comparator 4. The comparator 4 compares the delayed input analog signal VIN with the output from the converter 8. A comparison result determines the next bit which is then stored in a memory 7. The above operation is repeated to sequentially determine lower bits.
In the conventional A/D converter of this type, since comparators and D/A converters are used, the delay times of the delay circuits must be set with high precision. Errors in delay circuits degrade A/D conversion precision. Furthermore, in the conventional device, different reference voltages are required for the comparators, respectively, thus resulting in a complex circuit arrangement.